The recent proliferation of the use of computers and data processing has brought about a need for great quantities of electronic data storage devices and data storage media for use in such devices. An essential part of the manufacturing process of such media is certification. Certification is the general term for the processes of testing the media for defects and then classifying the media according to criteria such as defect quantity and location. The demands of the market and the industry, as compared to the present capabilities of manufacturing processes used to produce data storage media are such that nothing less than 100% testing of the media is acceptable. Furthermore, recent increases in required data density have created a need for even more stringent testing. Because a considerable expense in both time and equipment is involved in the 100% testing of media, it would obviously be desirable to find a way to reduce the time required for such testing. However, any method for speeding up the process cannot be allowed to interfere with the quality of the testing.
Numerous tests have been devised, each of which has been thought to provide some indication of the quality and/or usefulness of the media under test. These can be generally classified as parametric tests which are performed at certain designated locations on a medium and as defect scan tests which are performed on the entire recording surface. Two defect scan tests are the Missing Pulse Test and the Extra Pulse Test. These two test are thought to be essential basic tests for data storage media.
The Missing Pulse Test is a two step process involving first the writing of data onto a media at a maximum anticipated data density, and then the reading back of the just written data to verify that each of the just written data locations properly reproduces a pulse. Additionally, a "modulation" test, wherein a certifier looks for periodic fluctuations in pulse characteristics, can be performed simultaneously with the Missing Pulse Test. Additional tests, such as a "peak shift" test, wherein excessive deviation of the points of maximum amplitude of a signal from expected locations is the criteria of a media defect, may also be performed at this time. A test method wherein a "peak shift" type test is used as the primary means of detecting media defects is taught in U.S. Pat. No. 3,686,682 issued to Behr et al.
The Extra Pulse Test is, in many respects, an antithesis of the Missing Pulse Test. A medium, or areas of a medium, that should not contain data is checked to verify that no signal is present which might incorrectly be interpreted as a data bit.
Whether these tests are performed on an entire medium surface at a time, or upon sequential portions, current practice dictates the use of at least four process steps to accomplish the combination of the Missing Pulse Test and the Extra Pulse Test. Generally, an area is overwritten with data, then read to accomplish the Missing Pulse Test, then erased of all data (by a direct current signal, in the case of magnetic media), and then read to accomplish the Extra Pulse Test. U.S. Pat. No. 3,480,331, issued to the present inventor, has taught a method wherein eight process steps were used to accomplish these two tests, in order to improve the reliability of the tests.
Since the Extra Pulse Test and the Missing Pulse Test are thought to be necessary to be performed on every data location of every manufactured medium, most of the cost in time and expense incurred during media testing are a result just these two tests. Several methods have been developed in the prior art to speed up media testing. Examples are found in U.S. Pat. No. 4,746,995 issued to Rauskolb and in U.S. Pat. No. 3,781,835 issued to Dion et al. However, all of the prior art methods have either sacrificed some accuracy of the test (as, for instance, by not actually checking all specific data locations) or have not effectively sped up the process in application.
It should be noted that, while the Missing Pulse Test and the Extra Pulse Test are but two of a number of tests normally performed on data storage media, they are the tests which most directly correlate to actual usage criteria. The qualities of reproducing a pulse where one has been recorded, and of not producing a pulse where none has been recorded are the sine qua non of a properly functioning storage medium. Therefore, it might seem that it would be best to try to exactly duplicate the conditions of actual use during such testing. This is particularly true since, in actual usage, automatic gain control circuitry is used to compensate for variances in a particular medium and in such factors as variations in linear speed past a read/write head as a function of radial displacement on rotating media. In fact, that is essentially what is conventionally done for the Missing Pulse test. The data is first written to the medium by means closely approximating the actual operation of the "write" circuitry of an end user data storage device, and then is read back in a similarly conventional fashion. An average peak magnitude of the pulses obtained from a local area of the media can be derived, against which the individual pulses my be compared in order to ascertain if any of the individual pulses fall below an acceptable percentage of the local average. However, there are additional problems peculiar to the Missing Pulse Test. Primarily, the problem exists that on areas of a medium that are intentionally devoid of deposited signal, there is no "average" pulse amplitude against which a possible errant pulse might be compared to see if it exceeds a maximum error threshold. Indeed, this problem was one of those addressed by the present inventor's '331 patent, cited previously. Several other methods of solving this problem have been employed, including retaining local area averages obtained from a prior Extra Pulse Test for use in the Missing Pulse Test.
Obviously, it would be desirable to be able to test for Extra Pulse during the Missing Pulse portion of a test, without having to first DC erase the media. However, all of the prior art data media certification processes within the inventor's knowledge, which have attempted to test both for defects at individual data locations which could cause extra pulses and for defects at individual data locations which could cause missing pulses, have employed a multi step process to test for missing pulse and an additional process to test for extra pulse.
Yet another trend in the industry has increased the desirability of an improved test method. Due to the great quantities of hard disk media that is being manufactured, the makers of hard disk assemblies (HDA) have found it to be economical to forgo testing of media until the HDA is assembled. Completed HDA are tested at each data location after assembly. But it would be unrealistic to expect that there will be no faulty locations in an entire HDA having millions of individual locations. Therefore, a routing table, which differentiates usable and unusable locations, is made and stored at a designated location on the medium. Since this test must be performed, it is thought to be redundant to test the media before assembly. Of course, this means that if there are excessive media defects, a completed HDA may have to be rejected at final testing. However, as long as there are sufficiently few rejected units, it is more economical to just test the finished HDA, but this introduces additional considerations. One way to test the HDA is to simply write to it, and then to read back what has been written through the user access ports in the same manner that data will be written and then read during actual use. In fact, this is what is now frequently being done. Unfortunately, this test method allows locations that are only barely able to reproduce a data pulse to be deemed acceptable. As the machine ages and circuit characteristics change slightly, such locations may well eventually be a source of problems.
Clearly, it would be better to tap into the HDA circuitry at the analog level and to do a test similar to the media certification test described previously. However, because of the excessive amount of time that would be required to perform such a test using conventional methods, manufacturers have generally opted for the quicker digital output test. An additional reason for this choice is that the circuitry of many HDA will not allow for a DC erase, and thus a conventional EP test on such units is not possible.
A method which could provide a higher reliability test at the analog level and yet be run quickly enough to be economically feasible would obviously be desirable. However, no prior art certification process, to the inventor's knowledge, has been capable of meeting these criteria. Furthermore, no prior art media certification process, to the inventor's knowledge, has successfully reduced the number of process steps required for media certification while maintaining or improving the accuracy of the test. All successful applications to date have been a compromise between accuracy and speed.